The invention relates to a polymerization process for obtaining sulphonic fluorinated ionomers with high productivity.
Specifcally, the invention relates to a polymerization process in microemulsion for obtaining sulphonic fluorinated ionomers, said process characterized by high productivity and by operating conditions such to minimize the coagulum formation during the polymerization and the reactor corrosion.
The polymerization of fluorinated monomers in microemulsion is known in the prior art. It shows a series of advantages with respect to the conventional polymerization in emulsion. Generally higher polymerization yields are obtained.
In U.S. Pat. No. 4,864,006 it is described that the microemulsion can be diluted when it is introduced into the reaction medium, allowing to control the polymer particle nucleation and to determine the number thereof. In said patent sulphonic fluorinated monomers are not mentioned among those polymerized in microemulsion. Tests carried out by the Applicant (see the Examples) have shown that, in the case of polymerization of sulphonic fluorinated monomers, by operating under the conditions indicated in said patent, an increase of the productivity of the polymerization process is not obtained with respect to the case of emulsion polymerization. Besides, there are problems of reactor corrosion and stability of the obtained latex and coagulum formation during the polymerization. The coagulum formation, in the case of an industrial reactor, requires frequent stops of the reactor for the cleaning. Therefore in a continuous or semi-continuous process the productivity is furtherly reduced. This drawback due to the formation of coagulum, besides taking place under the operating conditions described in U.S. Pat. No. 4,864,006, takes place also in the case of emulsion polymerization of sulphonic fluorinated monomers.
The need was therefore felt to have available a polymerization process for obtaining sulphonic fluorinated ionomers, said process characterized by high productivity, absence of coagulum formation during the polymerization and by minimization of the reactor corrosion.
An object of the invention is a polymerization process in microemulsion of:
(I) one or more fluorinated monomers containing at least one ethylene unsaturation;
(II) one or more florinated monomers containing sulphonyl groups xe2x80x94SO2F; said process comprising:
a) preparation of an aqueous microemulsion, having a pH in the range 4-7, of perfluoropolyethers (PFPEs) having non reactive end groups or end groups optionally containing 1 or more H, Cl atoms instead of fluorine;
b) feeding of the microemulsion into the polymerization reactor, in an amount higher than 18 ml per liter of reaction medium, preferably 25-50 ml per liter of reaction medium;
c) reactor degassing, introduction of the monomer (II) in liquid form into the reactor, reactor pressurization with the gaseous monomers (I), optional addition of surfactants, stabilizers, chain transfer agents;
d) addition of the initiator, and during the polymerization feeding of the monomers (I) so as to maintain constant the reactor pressure and optionally further addition of the monomer (II) and of the chain transfer agents.
The latex discharged from the reactor can be subjected, if desired, to the usual post-treatments in connection with the considered uses, in particular to the coagulation and drying process.
The fluorinated monomers of type (I) are selected from:
vinylidene fluoride (VDF);
C2-C8 perfluoroolefins, preferably tetrafluoroethylene (TFE);
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example trifluoromethyl. bromodifluoromethyl, pentafluoropropyl;
CF2xe2x95x90CFOX perfluoro-oxyalkylvinylethers, wherein X is a C1-C12 perfluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.
The fluorinated monomers of type (II) are selected from:
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F (vinylsulphonylfluoride);
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94[CF2xe2x80x94CXFxe2x80x94O]nxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F
wherein Xxe2x95x90Cl, F or CF3; n=1-10;
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F;
F2Cxe2x95x90CFxe2x80x94Arxe2x80x94SO2F.
Optionally in the polymerization process of the invention, besides the monomers of type (I) and (II), bis-olefins of formula:
R1R2Cxe2x95x90CHxe2x80x94(CF2)mxe2x80x94CHxe2x95x90CR5R6xe2x80x83xe2x80x83(III) 
wherein m=2-10, preferably 4-8,
R1, R2, R5, R6, equal to or different from each other, are H or alkyl groups C1-C5,
can be used as comonomers, in an amount in the range 0.01%-5% by moles.
Optionally in the polymerization process of the invention brominated and/or iodinated cure-site comonomers can be fed, such bromo and/or iodo olefins having from 2 to 10 carbon atoms (as described for example in U.S. Pat. No. 4,035,565 and U.S. Pat. No. 4,694,045), or iodo and/or bromo fluoroalkylvinylethers (as described in U.S. Pat. No. 4,745,165, U.S. Pat. No. 4,564,662 and EP-199,138), in such amounts wherefore the content of cure-site comonomers in the final product is generally in the range 0.05-2 moles per 100 moles of the other basis monomeric units.
Alternatively or also in combination with the cure-site comonomers, it is possible to introduce in the end groups iodine and/or bromine atoms by addition to the reaction mixture of iodinated and/or brominated chain transfer agents, such for example the compounds of formula Rf(I)x(Br)y, wherein Rf is a (per)fluoroalkyl or a (per)fluorochloroalkyl having from 1 to 8 carbon atoms, while x and y are integers in the range 0-2, with 1xe2x89xa6x+yxe2x89xa62 (see for example U.S. Pat. No. 4,243,770 and U.S. Pat. No. 4,943,622). It is also possible to use as chain transfer agents iodides and/or bromides of alkaline or alkaline-earth metals, according to U.S. Pat. No. 5,173,553. Other usable chain transfer agents are compounds hydrogen containing, such as hydrocarbons, alcohols, in particular ethyl acetate and ethane.
The polymerization initiators used in the process of the present invention are preferably radical inorganic initiators, such for example ammonium and/or potassium and/or sodium persulphate, optionally in combination with ferrous, cuprous or silver salts. The initiator feeding procedures can be in a continuous way or with a single addition at the polymerization starting.
Preferably in the polymerization process of the present invention:
TFE;
CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2SO2F;
bis-olefin of formula (III), wherein R1, R2, R5/R6 are H and m=6 or 8 are used as comonomers.
The microemulsions used in the process of the present invention are described in U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283, herein incorporated by reference, wherein instead of the mentioned perfluoropolyethers having non reactive end groups, also hydrofluoropolyethers having one or both end groups containing one H atom, or having one or more chlorine atoms instead of fluorine in the chain end groups, can be used. The surfactants which can be used both for preparing the microemulsion and during the polymerization, are those described in the mentioned patents or those having an end group wherein one or more fluorine atoms are substituted by chlorine and/or hydrogen. The molecular weight of the perfluoropolyethers (PFPEs) which can be used can be lower also than 500, for example also 300 than as number average molecular weight.
The microemulsions used in the present invention have a pH in the range 4-7, obtained by suitably dosing the base to neutralize the surfactant agent. It has been in fact observed, see the Examples, that the use of a microemulsion having a pH in the range 4-7 minimizes the corrosion phenomena of the reactor.
The Applicant has surprisingly found that the necessary microemulsion amount to have the advantages of the present invention in terms of productivity and absence of coagulum formation, must be higher than 18 ml per liter of reaction medium, as shown in the Examples. Such amount is clearly higher than the upper limit indicated by the prior art (U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283). By using the microemulsion in the amounts indicated in the prior art, no advantage is obtained in terms of productivity and absence of the coagulum formation with respect the emulsion polymerization.
With the polymerization process of the invention it is possible to obtain sulphonic fluorinated ionomers having a high molecular weight and a good chemical and thermal stability. However, it is always possible to lower the molecular weight by using suitable chain transfer agents as above indicated.
With the polymerization process of the invention it is furthermore possible to obtain sulphonic fluorinated ionomers having both high and low equivalent weight, in particular in the range 380-1,300.
The polymerization reaction is generally carried out at temperatures in the range 25xc2x0-120xc2x0 C., under pressure, for example up to 3 MPa.
Preferably the polymerization process of the invention is carried out by using a microemulsion comprising:
a (per)fluoropolyether oil phase of formula:
CF3O(CF2xe2x80x94CF(CF3)O)n(CF2O)mCF3 
wherein n/m=20 having average molecular weight in the range 400-600,
a fluorinated surfactant of formula:
CF2ClO(CF2xe2x80x94CF(CF3)O)n(CF2O)mCF2COORxe2x80x2
wherein Rxe2x80x2xe2x95x90Na, K, NH4,
n/m=10 having average molecular weight in the range 500-700.
In the microemulsion containing as essential elements the (per)fluoropolyether oil phase, the fluorinated surfactant agent, water, and optionally co-surfactants as alcohols, small amounts of other compounds, such as for example chain transfer agents, initiators, functionalized comonomers, amorphous polymers such as for example homopolymers of perfluorodioxoles or copolymers of perfluorodioxoles with TFE, perfluorocarbons, etc., can be dissolved in the fluorinated oil phase.
Among the fluorinated surfactant agents suitable for obtaining the microemulsions of the present invention, anionic fluorinated surfactants agents, for example salts of fluorinated carboxylic acids or of sulphonic acids, having a perfluoro-polyether or perfluorocarbon structure or partially fluorinated, cationic surfactant agents, for example quaternary ammonium fluorinated salts, or still fluorinated non ionic surfactants agents, can be mentioned. The above surfactants can be also used in admixtures.
The sulphonic fluorinated ionomers obtained by the invention process are suitable to be used in the preparation of membranes for fuel cells, of membranes for electrochemical applications, such as for example chlorosoda cells, lithium batteries, and electrodialysis, and in reactors in which the ionomeric membrane has the function of superacid catalyst.
The following Examples are mentioned for illustrative, but not limitative purposes of the scope of the invention.